In the early years of aviation, all flight operations were conducted in visual meteorological conditions (VMC) since safe operation of an aircraft was only possible with reference to visual cues such as a visual horizon and geographical terrain. In subsequent years, various ground-based electronic navigational aids and aircraft cockpit instrumentation systems were developed that cooperatively permitted navigation in instrument meteorological conditions (IMC). Accordingly, it was possible to perform enroute navigation when the aircraft is above or within cloud layers where familiar visual references are absent. Somewhat later, ground-based systems, aircraft instrumentation and landing procedures were developed that permitted a suitably equipped aircraft to land at a suitably configured airport when the conditions at the airport would not allow operations by visual reference alone. For example, non-precision approach procedures are commonly available at many airports so that the aircraft may safely land under IMC with reference to a ground-based navigational aid such as a non-directional beacon (NDB) or a very high frequency omnirange (VOR) installation. In addition, precision approach procedures are commonly available at larger airports to permit higher volumes of traffic into the airport to be conveniently accommodated in IMC. For example, the well-known instrument landing system (ILS) permits a suitably equipped aircraft to controllably approach and descend to an airport runway by providing the aircraft with localizer information that assists the aircraft with lateral course guidance, and with glide slope information that permits the aircraft to controllably descend to the airport runway.
In general, ILS ground-based systems, aircraft instrumentation and landing procedures are organized into ILS Categories that characterize the airport conditions that the ILS system will support. Briefly and in general terms, the various ILS Categories each provide a decision height (DH), and a runway visual range (RVR). The DH specifies how close to the runway a pilot may descend while attempting to visually detect the runway, while the RVR describes the visual range the pilot may expect at the selected airport. Accordingly, before executing the ILS approach or before attempting to land, the pilot must verify that the airport RVR is currently equal to or greater than the minimum RVR as provided in the ILS procedure. If the RVR condition is satisfied, the ILS approach may be attempted, and the pilot is expected to observe the runway, and/or a runway lighting system when the aircraft reaches the DH. Otherwise, the pilot must abort the landing and execute a “missed approach” in conformity with the procedure. Currently, ILS Category I provides for approaches having a DH of not less than 200 feet, and an RVR of not less than 1,800 feet, while ILS Category II permits approaches to a DH of not less than 100 feet, with an RVR of not less than 1,200 feet. Still other ILS Categories provide for even lower minimum requirements. Example, ILS Category III generally provides no DH minimum, and an RVR of not less than 700 feet for Category IIIa; an RVR of not less than 150 feet for Category IIIb; and approaches without an RVR minimum for Category IIIc.
Accordingly, during airport operations in low visibility, the entire runway length is generally not visible to the pilot, so that the pilot is precluded from visually observing that other aircraft are not present on the runway, or approaching the runway. At airports where aircraft moving about the airport on taxiways adjacent to runway are under the control of a ground controller positioned in an airport control tower, the ground controller is limited to visual observations of the aircraft moving about the airport. Consequently, the ground controller must generally assume that aircraft are moving in compliance with the instructions issued by the controller, and rely on position reports from aircraft as they maneuver about the airport. The position reports are also available to other aircraft that are moving on the airport since a common radio frequency is used. Airports without ground controllers rely exclusively on pilots of taxing aircraft knowing where they are at all times.
Ground surveillance radar systems are available at selected airports to assist ground controllers when the airport visibility is limited. Images obtained by the ground surveillance radar system permit individual aircraft to be identified and further permits the movement of the aircraft to be tracked by the ground controller in real time. Although ground surveillance radar system constitutes an improvement in the state of the art, they are generally costly systems, and consequently, are available at relatively few airports. Furthermore, ground surveillance radar systems require the presence of a ground controller, who may not be present during selected hours unless the airport control tower is continuously maintained. Accordingly, the ground surveillance radar system, if present, may not be available when visibility conditions are poor due to a tower closure. Moreover, even if the radar system is present and continuously monitored by a ground controller, the controller may still issue erroneous instructions to a flight crew, or the flight crew may not properly comprehend proper instructions, which may contribute to an aircraft collision.
Therefore, due to the complexity of many airports, and further in view of the increasing traffic activity present at many airports, the possibility of operational errors is significantly increased during periods of low visibility. In particular, the possibility of a runway incursion by an unauthorized aircraft due to a missed position report or a communications error is significantly enhanced.
What is needed in the art is a system and a method that permits an aircraft to readily inform other aircraft that the aircraft is on the runway. Furthermore, the system and method should also permit the aircraft occupying the runway to communicate with aircraft executing a landing approach.